Declining immune function is well described in the elderly, and contributes significantly to increased risk and severity of infection, impaired responses to vaccination, and poorer control of cancer. Infections with several pathogens, whether viral (e.g., herpes viruses), bacterial (e.g., Mycobacteria) or parasitic (e.g., Leishmania, persist for the lifetime of the host. Reactivation of such persistent infections is a common consequence of waning immune function in the elderly. Regulatory T cells (Treg), a specialized subset of CD4+ T cells that have been shown to control the intensity of immune responses accumulate dramatically with age. While Treg negatively regulate immune responses, it is currently unclear whether their accumulation contributes negatively to immune function in the elderly. Our preliminary data in elderly humans and in a mouse model of Leishmania major infection show that Treg actively suppress the functionality of effector T cells. Our preliminary data also show that Treg depletion increases aged human cytomegalovirus (CMV)-specific T cell responses in vitro. In addition, in murine CMV (MCMV)- infected young mice, Treg depletion significantly increased MCMV-specific T cell responses in vivo. Although Treg likely contribute to immunosuppression with age, the mechanisms underlying Treg accumulation in the elderly remain unclear. Our preliminary data suggest that Treg accumulation in mice is progressive and is caused by their increased resistance to apoptosis. More specifically, we found that (i) levels o the pro-apoptotic molecule Bim in Treg are normally and progressively decreased with age; (ii) in the absence of Bim, Treg accumulate faster; (iii) IL-2 and IL-15 play a partially redundant role in driving Treg accrual in aged mice; (iv) the additional loss of Bim restores the loss of Treg in aged IL-15-deficient mice; (v) acute, in vivo loss of forkhead box transcription factor subclass O (FOXO) members 1, 3a, and 4 lead to decreased expression of Bim in Treg . Together, our data suggest a model in which chronic cytokine signaling increases in vivo survival of Treg by enhancing FOXO3a-dependent suppression of Bim-expression. Our overarching hypothesis is that accrual of Treg in aged mice and humans are due to cytokine-driven repression of Bim expression and contributes to immunosuppression and pathogen persistence. This hypothesis will be tested in three Specific Aims: (1): Define the mechanisms of increased accumulation of Treg in aged hosts; (2) Define the homeostasis and functionality of Treg in two models of persistent infection, Leishmania major and MCMV; (3) Determine if a Bim/FOXO axis controls Treg survival in humans and the role of Treg in dampening CMV-specific T cell responses in elderly humans. The long-term aim of this research is to determine the role of Treg in the immune suppression of aging, and to devise novel therapeutic approaches to enhance infection control as well as vaccine efficacy in the elderly.